20 ILLINOIS BIOLOGICAL MONOGRAPHS [466 



these two the second is the larger. In the parenchymatous nuclei the 

 nucleoli are quite small, not definite in outline, and much chromatin 

 is scattered thru the nucleus. 



The excretory vessel in the tail of Cercaria inhabilis is a large 

 single tube running down almost to the tip, where it divides to open 

 to each side (Fig. 16). This vessel becomes narrow as it passes from 

 the tail to the body but immediately widens into the excretory bladder, 

 which is a triangular vesicle dorsad and anteriad to the acetabulum. 

 No vessels or flame cells were made out in the acetabulum. Into 

 each corner of the bladder open vessels which were traced only as far 

 as the eye-spots. In these vessels were present the round highly refrac- 

 tive concretions, already noted in the excretory system of Cercaria 

 urbanensis and by Looss in the amphistome cercariae described by 

 him (Looss, 1896:181). 



The mouth of Cercaria inhabilis is at the anterior tip of the oral 

 sucker which juts out slightly from the body. The anterior margin 

 of the mouth cavity is smooth or only slightly roughened. There are 

 no papillae present around the margin of the oral cavity such as Looss 

 (1896:179) describes for the amphistome cercaria from Egypt, which 

 he considers, to be the larva of Gastrodiscus egyptiacus. The mouth 

 opens into an oral cavity, from which a narrow passage runs directly 

 backword; this changes into the esophagus at the posterior limit of 

 the oral sucker. Into each side of the oral cavity open the blind tubes 

 of the retrodorsal pharyngeal pockets (Fig. 16, pp.). The oral cavity 

 has a length of 0.059 mm. and the lateral blind tubes are 0.086 mm. 

 in length. As it passes out of the oral sucker the digestive tube becomes 

 the thin, straight-walled estfphagus, 0.021 mm. in caliber, which has a 

 length of 0.1 to 0.15 mm. depending on the state of contraction of 

 the cercaria. Just before the esophagus bifurcates it is reinforced by 

 a mass of circular muscle fibers of a sphincter, making a characteristic 

 structure which superficially resmbles a pharynx. This pseudo-pharynx 

 is about as long as wide, being 0.038 mm. in diameter, and has a wall 

 0.009 mm. thick, composed of from 12 to 14 separate layers of mus- 

 cles. Longitudinal muscles were not made out either in this region or 

 in the esophagus. The inner lining of the esophagus is non-cellular 

 and tiny projections extend out into its lumen (Fig. 17, es). After 

 bifurcation the intestinal ceca run laterally for a short distance and 

 then turn posteriad to reach within 0.05 mm. to 0.06 mm. of the anterior 

 margin of the acetabulum. The ceca are fairly wide, 0.028 mm. to 

 0.036 mm., and the inner walls contain flattened nuclei which jut out 

 a little into their lumina. Figure 16 illustrates the relations of the 

 digestive system. 



